1. Field of the Invention
The present invention relates to a composite material with graded function (hereinafter referred to as functionally gradient composite material) which consists principally of copper and carbon, and more particularly to a copper-carbon functionally gradient composite material suitable for use in a nuclear fusion reactor as materials for wall members or members of high heat flux portions which should be durable against irradiation of high energy electron beam, ion beam or high-speed neutrons, and a method for manufacturing the same.
2. Description of the Prior Art
Heretofore, as a wall member, particularly as a first wall member facing plasma in a nuclear fusion reactor, metal materials such as stainless steel or TiC-coated Mo have been used. These metals, however, tend to evaporate when heated by collision of plasma and to enter the plasma as impurities so that the radiation loss of plasma increases and the temperature of plasma falls.
To eliminate this disadvantage, carbon having the small atomic number, especially graphite have been used in order to reduce the radiation loss of plasma due to impurities. As the reactor wall material facing plasma, isotropic graphite material of high purity and having fine grain structure has been at the focus of attention, but cracks may often be caused in it due to thermal shock when subjected to excessive heat load. Accordingly, there has recently come into the limelight a carbon fiber-reinforced carbon (C/C) composite material having high strength, high thermal conductivity and desirable thermal shock resistance properties.
Such type of carbon material, however, lacks a heat-removing function and it starts evaporating or subliming when subjected to high-temperature heat load. Accordingly, it is necessary to let the carbon material be accompanied by a certain kind of cooling means. This is achieved in general by bringing the carbon material into contact with metal. For instance, brazing is used for uniting copper having high heat conductivity with graphite. This method, however, would require several kinds of metallic buffer materials and brazing filler metals because of the difference in the coefficient of thermal expansion between copper and graphite, and it also requires quite complicated procedures to provide an integral reactor wall member. Furthermore, due to the large difference in the coefficient of thermal expansion as mentioned above, this method is impractical for the integration into large-sized products and requires connection of segmented parts, which undesirably increases the number of parts, necessary labor and time.
Meanwhile, in order to make the carbon material have graded function, there have been used a chemical vapor deposition (hereinafter referred to as CVD) method, a matrix powder mix laminating method, or the laminating method described in Japanese Patent Laid-Open No. Hei 4-295055, and each has its own characteristics.
Among the aforementioned methods, the CVD method is performed by introducing one or more gases and depositing pyrolytic carbon or ceramic on a carbon substrate. In this case, the composition and characteristics of the produced material can be freely adjusted by varying the concentration of each gas. However, it requires a lot of time for the depositing procedure and accordingly it is industrially impractical.
In the powder mix laminating method, several layers of powder are laminated with a calculated particle size-volume ratio of two or more materials and are then shaped by hot press or the like, thereby enabling the production of relatively large-sized products. As a functionally gradient material, however, there are interfaces in proportion to the number of layers. In consequence, the function is not continuously gradient.
Further, in the laminating method described in Japanese Patent Laid-Open No. Hei 4-295055, a number of sheets are laminated and are then pressed, resulting in the stepwise change of composition as mentioned above, so that a uniform change of function does not continue. In order to make such gradient of function close to continuous one, considerable time and labor would be required. As a result, it would be quite impractical in industry.
The present inventors have recognized the necessity of developing materials having gradient composition of C/C composite material and copper as described in "Journal of Plasma and Fusion Research Vol. 69, No. 5(1993) at pp. 439 "Graphite (Carbon) material for Nuclear Fusion Reactor" written by Tetsuro Tanabe and Tadashi Maruyama, but it has not been possible to reach practical stage for realization.